The present invention relates to a switched capacitor adaptive line equalizer for use in digital communication over four-wire subscriber lines.
Prior art switched capacitor line equalizers (EQLs) using switched capacitor (SC) circuits are proposed in Kuraishi et al., "A Switched Capacitor Adaptive Line Equalizer for a High-Speed Digital Subscriber Loop" (hereinafter referred to as the Kuraishi paper), 1984 IEEE Custom Integrated Circuit Conference, pp. 264-268, and "Line Equalizer for a Digital Subscriber Loop Employing Switched Capacitor Technology", IEEE Transactions on Communications, Vol. COM-30, No. 9, September 1982. According to the Kuraishi paper, the .sqroot.f characteristic of the SC-EQL is varied by changing the capacitance in an SC filter. For this purpose, a capacitor array having a plurality of branch capacitors connected in parallel with switches is used.
In such an SC equalizer, the gain is usually shared with the .sqroot.f equalizer section and the other sections which have a capacitor array. For gain adjustment, the capacitance of each capacitor array is changed by switching the switches of each capacitor array. The switchings cause spiked noise and transient phenomena, resulting in a deterioration of the quality of the communication.
Additionally, since two-wire digital communication is a pingpong-like process in which transmission and reception alternately take place, the aforementioned problems will not arise. This is because the capacitances of the line equalizer on the transmitting side may be switched during transmission. Four-wire communication, on the other hand in which signals are constantly received, cannot avoid the adverse effects of noise and transient phenomena, which occur when branch capacitors are selectively switched to set the .sqroot.f characteristic.